1. Field of the Invention
The present invention relates to a magnetoresistive film made of a ferrimagnetic material whose major constituents are a rare earth metal and a transition metal, and more particularly to a magnetoresistive film that exhibits a relatively large magnetoresistive effect, a method of manufacturing the magnetoresistive film, and a memory that uses such a magnetoresistive film.
2. Related Background Art
In recent years, a semiconductor memory which is a solid state memory has been used extensively in information equipment. Semiconductor memories are in variety of types and include DRAMs, (dynamic random access memory), FeRAMs (ferroelectrics random access memory), and flash EEPROM (electrically erasable programmable read only memory). These semiconductor memories have advantages and disadvantages. No one type of semiconductor memory meets all requirements of information equipment in current use. For example, DRAMs have a high recording density and can be rewritten a large number of times. However, DRAMs are volatile, so that when the memory is turned off, the stored information is lost. Flash EEPROMs are non-volatile but take a long time for erasing information and therefore do not lend themselves to the high-speed processing of information.
In contrast to semiconductor memories, a memory based on magnetoresistive effect (MRAM: magnetic random access memory) is non-volatile and a potential material that will meet all the requirements of current information equipment including write time, read time, recording density, the number of times that information can be rewritten, and power consumption. Especially, an MRAM using a spin dependent tunnel magnetoresistance (TMR) provides a large read signal and is therefore advantageous in designing a memory of high recording density or high-speed reading. Recent researches showed the feasibility of a memory using the MRAM.
A basic configuration of a magnetoresistive film used as an MRAM element is a sandwich structure in which a non-magnetic layer is sandwiched between magnetic layers. Materials as the non-magnetic layer include, for example, Cu and Al2O3. A magnetoresistive film using, for example, Cu as a non-magnetic layer is referred to as GMR (giant magneto-resistance) film while a magnetoresistive film based on insulators such as Al2O3 is referred to as a spin dependent tunnel magnetoresistive (TMR) film. Typically, TMR films have larger magnetoresistances than GMR films.
FIGS. 6A and 6B illustrate a magnetoresistive film of a configuration where two magnetic layers are laminated with a non-magnetic layer sandwiched therebetween. Arrows indicate the direction of magnetization of the respective magnetic layers. As shown in FIG. 6A, if the magnetization directions of the two magnetic layers are oriented parallel, the electrical resistance (electrical resistance between the two magnetic layers) of the magnetoresistive film is relatively low. If the directions of magnetization are anti-parallel as shown in FIG. 6B, the electrical resistance is relatively high. Thus, by using one of the two magnetic layers as a memory layer and the other as a detecting layer and making use of the aforementioned property, reading information can be accomplished. For example, a magnetic layer 13 located on a non-magnetic layer 12 operates as a memory layer and a magnetic layer 11 under the non-magnetic layer 12 operates as a detecting layer. The information stored is assumed to be xe2x80x9c1xe2x80x9d when the magnetization direction of the memory layer (magnetic layer 13) is rightward and xe2x80x9c0xe2x80x9d when the magnetization direction is leftward.
If the magnetization directions of the magnetic layers 11 and 13 are both rightward as shown in FIG. 7A, the electrical resistance of the magnetoresistive film is relatively low. If the magnetization direction of the magnetic layer 11 is rightward and the magnetization direction of the magnetic layer 13 is leftward as shown in FIG. 7B, the electrical resistance of the magnetoresistive film is relatively high. Likewise, if the magnetization direction of the magnetic layer 11 is leftward and the magnetization direction of the magnetic layer 13 is rightward as shown in FIG. 7C, the electrical resistance of the magnetoresistive film is relatively high. If the magnetization directions of the magnetic layers 11 and 13 are both leftward as shown in FIG. 7D, the electrical resistance of the magnetoresistive film is relatively low. When the magnetization direction of the detecting layer 11 is rightward, a relatively high electrical resistance indicates that the memory layer 13 holds xe2x80x9c0xe2x80x9d and a relatively low electrical resistance indicates that the memory layer 13 holds xe2x80x9c1xe2x80x9d. Alternatively, when the magnetization direction of the detecting layer 11 is leftward, a relatively high electrical resistance indicates that the memory layer 13 holds xe2x80x9c1xe2x80x9d and a relatively low electrical resistance indicates that the memory layer 13 holds xe2x80x9c0xe2x80x9d.
Thus, the constituents of the magnetic layers 11 and 13 are selected such that the detecting layer 11 has a relatively large coercive force and the memory layer 13 has a relatively small coercive force. Information can be written by magnetizing the detecting layer 11 in one direction and applying a magnetic field to the memory layer 13 in such a way that the memory 13 is changed in the direction of magnetization but the detecting layer 11 is not reversed in the direction of magnetization. Information can be read by detecting the electrical resistance of the magnetoresistive film.
MRAMs use a surface magnetization film as a magnetic layer. If the size of the magnetoresistive film is made smaller in an attempt to increase the recording density of MRAM, the magnetoresistive film cannot hold information because of the demagnetizing field or curling of magnetization of the end surface of the element. One way of avoiding this problem, for example, is to make the magnetic layer rectangular. However, this approach cannot increase the recording density significantly because the element size cannot be smaller.
As described in, for example, U.S. Pat. No. 6,219,275, the Applicant of the present invention has proposed the use of a vertically magnetizing film to solve the aforementioned problem. The use of the vertically magnetizing film will not increase the demagnetizing field even if the element size is made smaller. Thus, the use of a vertically magnetizing film can implement a magnetoresistive film having a smaller size than a MRAM that uses a surface magnetizing film.
Just as in a magnetoresistive film using a surface magnetizing film, a magnetoresistive film using a vertically magnetizing film exhibits a relatively low resistance if the two magnetic layers are magnetized in parallel directions, and a relatively high resistance if the two magnetic layers are magnetized in anti-parallel directions. The magnetic layer 23 formed on the non-magnetic layer 22 is used as a memory layer and the magnetic layer 21 formed under the non-magnetic layer 22 is used as a detecting layer. The information stored in the magnetoresistive film is assumed to be xe2x80x9c1xe2x80x9d if the memory layer 23 is magnetized upward and xe2x80x9c0xe2x80x9d if the memory layer 23 is magnetized downward. When the magnetic layers 21 and 23 are both magnetized upward as shown in FIG. 8A, the electrical resistance of the magnetoresistive film is relatively low. When the detecting layer 21 is magnetized downward and the memory layer 23 is magnetized upward as shown in FIG. 8C, the electrical resistance of the magnetoresistive film is relatively high. Likewise, when the detecting layer 21 is magnetized upward and the memory layer 23 is magnetized downward as shown in FIG. 8B, the electrical resistance of the magnetoresistive film is relatively high, and when both the magnetic layers 21 and 23 are magnetized downward as shown in FIG. 8D, the electrical resistance is relatively low. In other words, when the detecting layer 21 is magnetized upward, a relatively high electrical resistance indicates that the memory layer 23 holds xe2x80x9c0xe2x80x9d and a relatively low electrical resistance indicates that the memory layer 23 holds xe2x80x9c1xe2x80x9d. Alternatively, when the detecting layer 21 is magnetized downward, a relatively high electrical resistance indicates that the memory layer 23 holds xe2x80x9c1xe2x80x9d, and a relatively low electrical resistance indicates that the memory layer 23 holds information xe2x80x9c0xe2x80x9d.
The vertically magnetizing film used in the aforementioned magnetoresistive element includes a film of alloy made of at least one element selected from rare earth metals such as Gd, Dy, Tb and at least one element selected from transition metals such as Co, Fe, and Ni, an artificial lattice film, a film of alloy made of transition metals such as Co/Pt and a precious metal, and a film of alloy of CoCr having a magneto-crystalline anisotropy in a direction perpendicular to the film surface. The film of alloy of rare earth metal and transition metal is a magnetoresistive film most suitable for a memory because it provides a magnetization curve having a rectangular ratio of 1, abrupt magnetic inversion when a magnetic field is applied to the film, and ease of manufacture.
The magnetoresistance ratio of a magnetoresistive film highly depends on the material in contact with a non-magnetic layer (tunneling barrier layer). It has been known from the past researches that Fe, Co, or alloys of Fe and Co exhibit large magnetoresistance ratios. However, the inventors of the present invention found that the magnetoresistive film shows a smaller magnetoresistance ratio when the magnetic material in contact with the non-magnetic layer is formed of a rare earth metal and a transition metal than when the magnetic material is formed of Fe, Co, or an alloy of Fe and Co. This can be due to the rare earth metal being adjacent the non-magnetoresisitive film. In other words, this can be to the fact that the atomic structure of rare earth metals in an alloy does not allow the rare earth metals to contribute to magnetoresistive effect and the electrons that conduct the atoms of rare earth metals existing in the interface between the tunnel barrier layer does not perform spin dependent tunneling.
The present invention was made in view of these drawbacks. An object of the invention is to provide a magnetoresistive film that uses a ferrimagnetic material whose major constituents are a rare earth metal and a transition metal and which exhibits a relatively large magnetoresistive effect. An object of the invention is to provide a method of manufacturing a magnetoresistive film, and a memory that uses such magnetoresistive films.
The magnetoresistive film according to the present invention is characterized in that the magnetoresistive film has first and second magnetic layers, a tunnel barrier layer sandwiched between the first and second magnetic layers, at least one of the first and second magnetic layers being a ferrimagnetic layer whose main constituents are a rare earth metal and a transition metal. A rare earth metal existing near the interface between the ferrimagnetic layer and the tunnel barrier layer is oxidized.
Rare earth metals such as Gd, Dy, and Tb are apt to be oxidized, and the oxides of these metals exhibit a higher electric resistivity than their non-oxidized atoms. Therefore, oxidizing a rare earth metal near the interface between the tunnel barrier layer and the ferrimagnetic layer will increase the magnetoresistive ratio effectively.
In the present invention, xe2x80x9ca ferrimagnetic layer whose major constituents are a rare earth metal and a transition metalxe2x80x9d refers to a magnetic layer that is primarily formed of a rare earth and a transition metal other than a rare earth metal and exhibits ferrimagnetism. The rare earth metal advantageously used in the invention is at least one element selected from the group of Gd, Dy, and Tb. The transition metal (other than rare earth metal) advantageously used in the invention is at least one element selected from iron group metals i.e., Fe, Co, and Ni.
The followings are methods of manufacturing magnetoresistive film.
A ferrimagnetic layer whose major constituents are a rare earth and a transition metal is formed, and then a tunneling barrier layer is formed on the ferrimagnetic layer. Then, the surface of the tunneling barrier layer is subjected to an oxidation treatment, thereby selectively oxidizing the atoms of a rare earth metal material of the ferrimagnetic layer that exists near the interface between the tunneling barrier layer. Rare earth metals are apt to be oxidized more easily than transition metals such as Fe, Co, and Ni. Thus, if an alloy of a rare earth metal and a transition metal is subjected to oxidation, the rare earth metal is selectively oxidized. Techniques of oxidation include, for example, plasma oxidation and natural oxidation. Any oxidation technique can be used. The tunneling barrier layer may not necessarily be formed after the ferrimagnetic layer is formed. For example, a material e.g., Al which can be a tunneling barrier layer through oxidation treatment may be formed and then the structure is subjected to oxidation where the selective oxidation of the rare earth metal and the formation of the tunneling barrier layer are performed simultaneously. Moreover, in order to oxidize the atoms of rare earth metal, the surface of the ferrimagnetic layer is subjected to oxidation before the tunneling barrier layer is formed and then the tunneling barrier layer may be formed.